Synthesis, crystal structure and thermal properties of di-μ-iodido-bis[bis(2-chloropyrazine-κN)copper(I)]

In the crystal structure of the title compound, the copper(I) cations are each tetrahedrally coordinated by two 2-chloropyrazine ligands and two iodide anions and linked into binuclear complexes by pairs of μ-1,1-bridging iodide anions.


Chemical context
Coordination compounds based on transition-metal halides show a versatile structural behavior, which is observed particularly in compounds that contain Cu I cations (Kromp & Sheldrick, 1999;Peng et al., 2010;Li et al., 2005;Nä ther & Jess, 2004). These compounds are also of interest because of their luminescence behavior (Gibbons et al., 2017;Mensah et al., 2022). For one given metal halide CuX (X = Cl, Br, I) and one specific neutral coligand, several compounds are usually observed that differ in the ratio between the metal halide and the coligand -this is the reason why so many compounds with different CuX (X = Cl, Br, I) substructures (such as, for example, dimers, single and double chains or layers) are observed that can be further connected into more condensed networks if bridging neutral coligands are used in the synthesis. In general, it is observed that with decreasing amounts of the coligand, the synthesis leads to the formation of compounds with more condensed CuX substructures. In this context, it is noted that upon heating, the most coligand-rich compounds usually lose their coligands stepwise and transform into coligand-deficient phases and that this is not limited to Cu I , but can also be expanded to Cd II and Zn II compounds (Nä ther et al., 2001(Nä ther et al., , 2007(Nä ther et al., , 2017Nä ther & Jess, 2001). This can easily be investigated by thermogravimetry of the most coligand-rich compounds, where each mass loss corresponds to the formation of a new coligand-deficient phase with a more condensed CuX substructure. Surprisingly, even for compounds with the same CuX:ligand ratio, sometimes a different thermal reactivity is observed. This is the case, for example, for compounds based on CuX (X = Cl, Br, I) and 2-chloropyrazine as ligands with the general composition CuX(2-chloropyrazine) (X = Cl, Br, I;Nä ther, Wriedt & Jess, 2002;Nä ther, Greve & Jess, 2002). In the isotypic chloride and bromide compounds, the copper cations are tetrahedrally coordinated by two bridging 2-chloropyrazine ligands and two halide anions. The cations are linked by single -1,1-bridging halide anions into chains that are further connected into layers by -1,4-bridging 2-chloropyrazine ligands (Fig. S1 in the supporting information). In contrast, in CuI(2-chloropyrazine), each copper cation is tetrahedrally coordinated by three iodide anions and only one terminal N-bonding 2-chloropyrazine ligand that is coordinated to the copper center by the N atom that is not adjacent to the chloro substituent. The cations are linked into double chains via bridging iodide anions (Fig. S1). If the chloride and the bromide compounds are heated, all 2-chloropyrazine ligands are removed in a single step, leading to the formation of CuX (X = Cl, Br). In contrast, the iodide compound decomposes in two discrete steps, where in the first step only half of the coligands are removed, leading to the formation of (CuI) 2 (2chloropyrazine), which decomposes into CuI upon further heating (Nä ther, Greve & Jess, 2002).
Concerning the composition of all of these compounds, in principle, more 2-chloropyrazine-rich compounds with the composition CuX(2-chloropyrazine) 2 might exist, in which, according to simple chemical considerations, each two copper cations would be tetrahedrally coordinated by two halide anions and two N-terminal 2-chloropyrazine ligands and linked into binuclear complexes by pairs of -1,1-bridging halide anions. One might argue that this arrangement is less stable compared to that with bridging 2-chloropyrazine ligands, but one should keep in mind that both N atoms of this ligand are not equivalent, because coordination to the N atom that is adjacent to the chloro substituent is sterically hindered. That this coordination exists is obvious from the crystal structure of (CuI)(2-chloropyrazine) mentioned above, even if this CuX substructure is different. Moreover, a few compounds with such a structure have already been reported in the literature, including, for example, (CuI) 2 (2-cyanopyrazine) 4 (Refcodes: DINQIA and DINQIA01; Jana et al., 2016), (CuI) 2 (2-ethyl-pyrazine) 4 (Refcode: EMELEN;Nä ther et al., 2003), (CuI) 2 -(methylsulfanylpyrazine) 4 (Refcode: QOWYOT; Artem'ev et al., 2019) and (CuI) 2 (2,2 0 -biquinoxaline) (Refcode: RIXGEL; Fitchett & Steel, 2008), all with iodide as counter-anion.
To check if such a compound can be synthesized, all three copper(I) halides were reacted in different solvents with a very large excess of 2-chloropyrazine, but no new crystalline phases were observed. On the contrary, if CuI is reacted as a suspension in pure 2-chloropyrazine, yellow-colored crystals of a new crystalline phase are obtained. In contrast, with CuCl or CuBr only the known compounds CuX(2-chloropyrazine) with X = Cl, Br are obtained. Single-crystal structure analysis proved that a new compound with the composition (CuI) 2 (2chloropyrazine) 4 has been obtained.

Structural commentary
The asymmetric unit of the title compound (CuI) 2 (2-chloropyrazine) 4 consists of one copper(I) cation, one iodide anion and two 2-chloropyrazine ligands that are located in general positions. The copper(I) cations are tetrahedrally coordinated by two symmetry-related iodide anions and two crystallographically independent 2-chloropyrazine ligands (Fig. 1). Each two copper(I) cations are linked by pairs of -1,1-bridging iodide anions into binuclear complexes consisting of fourmembered (CuI) 2 rings located on centers of inversion. The Cu-Cu distance within these rings amounts to 2.5643 (10) Å (Table 1). Bond lengths and angles are similar to those in related compounds and show that the tetrahedra are strongly distorted (Table 1).

Supramolecular features
In the crystal structure of the title compound, the binuclear complexes are arranged in columns that propagate along the crystallographic a-axis direction (Fig. 2). No directional intermolecular interactions occur between the complexes. One C-HÁ Á ÁN and one C-HÁ Á ÁI contact is observed, but their distances and angles indicate that they do not correspond to significant interactions (Table 2).

Powder X-ray diffraction and thermoanalytical investigations
Further investigations prove that the unreacted 2-chloropyrazine cannot be removed by filtration and washing because immediate decomposition is observed. Nevertheless, XRPD investigations reveal that most of the sample consists of crystals of the title compound, even if all of the powder patterns are of very low quality, which can be traced back to the instability of this compound and to the fact that only very small amounts of crystals were obtained and these were embedded in pure 2-chloropyrazine and that grinding of such samples leads to the formation of an amorphous phase (Fig. S2). Careful inspection of the powder pattern indicates that this sample is contaminated at least with CuI(2-chloropyrazine) reported in the literature (Nä ther, Greve & Jess, 2002). This indicates that the title compound has already decomposed into this compound at room temperature. To prove this assumption, freshly prepared crystals were stored at room temperature overnight and were afterwards investigated by PXRD, confirming that the title compound has been completely transformed into the ligand-deficient compound CuI(2-chloropyrazine) (Fig. S3). These observations indicate that CuI(2-chloropyrazine) with a bridging coordination of the 2-chloropyrazine ligand is more stable than the title compound, in which the 2-chloropyrazine acts as a terminal ligand. Additional DTA-TG-MS investigations reveal that the title compound loses two 2-chloropyrazine ligands in two subsequent steps, in which 2-chloropyrazine is always removed (m/z = 114, Fig. 3). The experimental mass loss in the first step (Ám exp . = 37.5%) is much larger than that expected for the Crystal structure of the title compound in a view along the crystallographic a-axis. Table 2 Hydrogen-bond geometry (Å , ).  (4)  127 Symmetry code: (ii) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 3
DTG, TG, DTA and MS trend scan curves for the title compound measured with a heating rate of 4 C min À1 . removal of all of the 2-chloropyrazine ligands from the title compound (Ám calc. = 19.1%), which originates from the fact that the 2-chloropyrazine coating the crystals cannot be removed. However, PXRD of the residue obtained after the first mass loss confirms that CuI(2-chloropyrazine) is formed as an intermediate (Fig. 4). It is noted that no additional step is observed that would correspond to the formation of the most 2-chloropyrazine-deficient compound, (CuI) 2 (2-chloropyrazine), because this event would happen at a much lower temperature, whereas our measurements indicate an excess of 2-chloropyrazine is still present in the gas phase. Finally, the product formed after the second mass loss was also investigated py PXRD, which proves that CuI (Hull, & Keen, 1994) is formed in this step (Fig. S4). Some related compounds can also be found with 2-methylpyrazine as coligand because the exchange of a chloro atom by a methyl group sometimes leads to compounds with similar crystal structures as the van der Waals radius of a chlorine atom is comparable to that of a methyl group (Desiraju & Sarma, 1986). This is obvious from CuX(2-methylpyrazine) with X = Cl, Br (Refcodes: XEBMOG and XEBMIA; Rossenbeck & Sheldrick, 2000), in which the copper(I) cations are linked by -1,1-bridging halide anions into chains that are further connected into layers by bridging 2-methylpyrazine ligands. This structure is identical to that of CuX(2-chloropyrazine) (X = Cl, Br). Moreover, both the 2-methylpyrazine and the 2-chloropyrazine compounds crystallize in the monoclinic space group P2 1 /c with very similar lengths of the unit-cell axes, but with a significantly different angle. In this context it is noted that with 2-methylpyrazine, two coliganddeficient compounds with the composition (CuX) 2 (2-methylpyrazine) with X = Br, I (Refcodes: XEBMUM and XEBNAT; Rossenbeck & Sheldrick, 2000) were observed that could not be prepared with 2-chloropyrazine.

Synthesis
CuI and 2-chloropyrazine were purchased from Sigma-Aldrich and used as received.
Yellow-colored single crystals suitable for single-crystal X-ray analysis were obtained within three days by the reaction of 0.5 mmol (95.23 mg) of CuI and 2 mL of 2-chloropyrazine. No stoichiometric ratios can be used as an excess of 2-chloropyrazine is needed because it acts as reactant and solvent. The additional 2-chloropyrazine cannot be removed by filtration and washing, because this leads immediately to the transformation of the title compound into CuI(2-chloropyrazine).

Experimental details
The data collection for single crystal structure analysis was performed using an Imaging Plate Diffraction System (IPDS-1) from Stoe with Mo K radiation. The PXRD measurements were performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator.
Differential thermoanalysis and thermogravimetry coupled to mass spectrometry (DTA-TG-MS) investigations were performed with a STA-429 thermobalance from Netzsch with skimmer coupling to a quadrupole mass spectrometer from Balzers. The measurements were performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles with a heating rate of 4 C min À1 . The instrument was calibrated using standard reference materials.

Di-µ-iodido-bis[bis(2-chloropyrazine-κN)copper(I)]
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.